The present invention relates generally to telecommunication systems and in particular, to transmission methods and devices in a wireless telecommunication system including at least a base station intended to communicate with terminals over bidirectional communication channels.
Note that wireless telecommunication systems include mobile telecommunication systems in which mobile terminals may move over long distances and sometimes quickly but also telecommunication systems in which the terminals are fixed or may only move over short distances relative to the base station which they are connected to and often very slowly.
FIG. 1 diagrammatically shows a wireless cellular telecommunication system serviced by plural base stations. In the FIG. 1 only one base station BTS is depicted, the base station BTS intends to communicate with at least one terminal, here three terminals TE1, TE2 and TE3, over wireless communication channels Ch1 to Ch3 respectively.
In the FIG. 1, the terminal TE1 is at a distance d1 from the base station BTS, the terminal TE2 is at a distance d2 from the base station BTS and the terminal TE3 is at a distance d3 from the base station BTS. The area covered by a base station BTS is generally called a cell 15, the border of said cell being at a distance of the base station considered as maximal.
Each channel Chi, with i=1 to 3, is intended to support an uplink channel UL for carrying information from the terminal TEi to the base station BTS and a downlink channel DLi for carrying information from the base station BTS to the terminal TEi. Said information is enclosed within frames split into time slots or sub frames allocated either to the uplink channel ULi or to the downlink channel DLi.
The frame is for example of the type depicted in FIG. 4, i.e. of the HD/OFDM type (standing for Half Duplex/Orthogonal Frequency Division Multiplex) either TDD/OFDM (Time Division Duplex/OFDM) or FDD/OFDM (Frequency Division Duplex). As it can be seen at FIG. 4, this frame is subdivided into an integer number L of time slots or sub frames TS1 to TSL that can be allocated either to the downlink channel DL or to the uplink channel UL. Furthermore, each sub frame TSj (j=1 to L) supports qj symbols s1 to sqj (here, for the sub frame TSj, qj=4), called OFDM symbols, respectively carried by k orthogonal modulation frequencies f1 to fk.
Note that each of the OFDM symbols s1 to sqj in a sub frame TSj generally includes a cyclic prefix that is used to combat inter-symbol interference.
It must be understood that in a general case the number of symbols per sub frame can vary from a sub frame to another.
In relation with FIG. 5 let's consider the transmission at time te of qj=four symbols s1 to s4 over the downlink channel DL by a base station BTS. These symbols s1 to s4 are received by a terminal TE1 at the border of the considered cell 15 (at a distance d1 from the base station BTS) at a time equal to te+RTD(d1)/2, where RTD(d1) is the Round Trip Delay for that terminal TE1 at said distance d1 from the base station BTS. These symbols are processed by the terminal TE1 which then transmits also symbols over the uplink channel UL. Before transmitting symbols over the uplink channel UL, a terminal TE1 has to wait for a period of time, said Receive Transmit Switch time or simply switching time and referred to as RTS, in order to take into account the duration of hardware and software operations. For instance, this delay RTS is the maximum of the time needed by hardware equipments of the terminals TE to switch between reception and transmission modes and the time needed by hardware equipments of the base station BTS to switch between transmission and reception modes. The symbols transmitted over the uplink channel UL are received at the base station BTS at a time tr equal to te+RTD(d1)+RTS+DDL, DDL being the total duration of the qj symbols. It can thus be seen that the base station BTS has to wait for the reception of the symbols transmitted by a terminal TEi located at the border of the cell 15 in order to perform the processing thereof. The waiting time is called the Guard Period GP, or idle period and must be equal at least to the round trip delay RTD(d1) plus the Receive Transmit Switch time RTS.
When a single terminal is involved in the present invention, it is named terminal TEi, with i=1 or 2 or 3 and so on up to the maximum number of terminals comprised in the coverage area of the Base station BTS.
When at least two terminals are involved in the present invention, they are named terminals TE.
Guard periods GP between downlink channels DL and uplink channels UL can be seen on FIG. 4.
The base station BTS determines a timing delay TD(d) for each of the terminals TE. The base station BTS transfers symbols to the terminals TE which transfer in response symbols to the base station BTS. These symbols are as example reference signals.
The timing delay is calculated using the following formula:TD(d)=tr−te−DDL−RTD(d)=GP−RTD(d),where d is the distance between each terminal TEi and the base station BTS.
From each timing delay, the base station BTS determines the Timing Advance TA=GP−TD(d) for each terminal and transfers the Timing Advance to the corresponding terminal TEi.
Each terminal applies its Timing Advance value for the transmission of symbols over the uplink channel UL in such a manner that the transmitted symbols are received at the base station BTS from all the terminals TE connected thereto at the same time tr.
The problem addressed by the telecommunication system afore described is related to a potential loss of resources resulting from the fact that during the guard period GP no information of any sort is transmitted or received at the base station.
In order to solve that problem, the inventors of the present invention have proposed in the European patent application EP 05291972 a new transmission scheme of information in the downlink channel or in the uplink channel.
In the patent application EP05921972, the base station BTS transmits at least a supplementary downlink symbol during the guard period to terminals TE that can receive said at least supplementary downlink symbol thereof and/or the base station BTS receives during the guard period at least a supplementary uplink symbol from the terminals TE that can transmit said at least supplementary uplink symbols during the time delay thereof.
Such technique is described in more details in reference to the FIG. 6.
In the FIG. 6, nref symbols s1 to s4 are transferred in a nominal part of a sub frame over the downlink channel DL by the base station BTS at a time referred to as te.
The nominal part of a downlink sub frame is the total duration of the qj symbols which can be transmitted to terminals TE which are located at the border of the cell or in other words to any terminal located in the cell of the base station BTS.
The nominal part of an uplink sub frame is the total duration of the qj symbols which can be transmitted by the terminals TE which are located at the border of the coverage area of the base station BTS.
After having transmitted the last downlink symbol s4 of the nominal part of a sub frame, the base station BTS has to wait during the guard period GP, up to time tr, for receiving uplink symbols from all the terminals TE connected thereto. The duration of the nominal part of a sub frame in the downlink channel is referred to as Dref corresponding to reference number nref of symbols, for example four.
The base station BTS is provided for including supplementary downlink symbols in a downlink sub frame, said supplementary downlink symbols being intended to be transmitted only to the terminals TE that can receive and process them during the respective time delay thereof.
If for a terminal situated at a distance d from a base station BTS, the time delay TD(d) is comprised between the duration of a number ndl of downlink symbols and the duration of a number ndl+1 of downlink symbols, respectively plus the switching time RTS, the base station BTS can insert information for that terminal in ndl supplementary downlink symbols. This condition can be mathematically written as follows:if ndl·tsdl≦TD(d)−RTS<(ndl+1)tsdl then insert at most ndl supplementary symbols.
tsdl being the duration of one downlink symbol.
When inserting information for a terminal TEi in ndl supplementary downlink symbols, the base station BTS indicates this insertion to that terminal TEi (by way of signalling) in order to enable the terminal TEi to read and process this ndl supplementary symbol or these ndl supplementary symbols along with the other symbols comprised in the nominal part of the downlink sub frame.
The base station BTS informs each terminal TEi connected thereto about the time delay TD or the Timing Advance it has to apply. Then, each terminal TEi, by using the just above expression, deduces from the value of the time delay TD or from the value of the Timing Advance the number of symbols that it has to read and to process.
The number ndl of supplementary downlink symbols that the base station BTS can allocate to a terminal TEi at a distance d of the base station BTS is thus determined in the following way:ndl=integer{(TD(d)−RTS)/tsdl}=integer{(GP−RTD(d)−RTS)/tsdl}
The maximum number Ndlmax of supplementary symbols is given for a terminal TEi that would be located at a zero distance from the base station BTS and for which the round trip delay RTD is zero:Ndlmax=integer{(GP−RTS)/tsdl}
Similar formulas as the above mentioned formulas are used for supplementary uplink channels.
As example, in the case depicted in FIG. 6, as the number Ndlmax is two, the total number of transmitted downlink symbols is now equal to four downlink symbols s1 to s4 of the nominal part Dref of a sub frame and two supplementary downlink symbols s5 and s6 that are transmitted in the period that is usually considered as a guard period GP. The value of the time delay TD(d1) for a terminal TE1 at the border of the cell 15, is equal to the switching time RTS in virtue of the definition of the time delay. Only the four downlink symbols s1 to s4 of the nominal part Dref of a sub frame are allocated by the base station BTS to said terminal TE1. The terminal TE1 only reads and processes those four downlink symbols s1 to s4, the two supplementary symbols s5 and s6, if any, being ignored or not processed.
The value of the time delay TD(d2) for the terminal TE2 is smaller than the duration of two downlink symbols plus the switching time but is however equal to the duration of one downlink symbol plus the switching time RTS. So, the base station BTS can transmit information to that terminal TE2 within at most one supplementary downlink symbol (here the downlink symbol s5 which follows the last downlink symbol s4 of the nominal part Dref of a downlink sub frame) which is read and processed by said terminal TE2. Symbol s6, if any, is ignored by said terminal TE2 or not processed. In this case, the total number of downlink symbols that can include information for that terminal TE2 is five (the four of the nominal part Dref of a sub frame s1 to s4 plus one supplementary symbol s5).
According to the example of the FIG. 6, the value of the time delay TD(d3) for the terminal TE3 is equal to the duration of two downlink symbols plus the switching time RTS. So, the base station BTS can transmit information to that terminal TE3 within at most two supplementary downlink symbols s5 and s6 which are read and processed by said terminal TE3. The total number of downlink symbols that include information for that terminal TE3 is six (four of the nominal part Dref of a sub frame s1 to s4 plus two supplementary downlink symbols s5 and s6).
The base station BTS can transmit information to any terminal TEi located at a distance d of the base station comprised between the distance d2 of the terminal TE2 and the distance d3 of the terminal TE3 with at most five downlink symbols that it can read and process. In the same way, the base station BTS can transmit information to any terminal located at a distance d shorter than the distance d3 of the terminal TE3 within at most six downlink symbols that it can read and process. Always in the same way, the base station BTS can transmit information to any terminal located at a distance d larger than the distance d2 of the terminal TE2 within at most four symbols that it can read and process.
Note that the downlink symbol s5 contains information only for the terminals TE that are located at a distance from the base station BTS lower than d2 whereas downlink symbol s6 contains information only for the terminals TE that are located at a distance from the base station BTS lower than d3.
When a terminal TEi gets connected to the base station BTS, no information about the time delay TD it has to apply has been received. As long as it is not done, the number of symbols allocated to this terminal TEi is equal to the reference number nref i.e. the number of symbols allocated to this terminal TEi is equal to the number of symbols comprised in the nominal part Dref of the sub frame. Furthermore, the terminal TEi transmits in the uplink with a predefined time delay, for instance equal to the guard period GP or to RTS, after receiving a number of symbols equal to the reference number nref.
When the coverage area 15 of the base station BTS is large, i.e. the round trip delay RTD is large for some terminals TE located at the border of the coverage area 15, few symbols are received or transferred by these terminals TE. It is then difficult to include a large number of pilot sequence elements in the symbols comprised in the nominal part of a sub frame for the purpose of analysing the channel conditions which exist between the base station BTS and the terminals TE without decreasing in an important manner the quantity of data transfer through the uplink or downlink channels.